[Amidala]

I read in the 3E rules tha a rouge gets Sneak attack.
What I got from the rules is this:
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Flanking
If a character is making a melee attack against an opponent, and an ally directly opposite the character is threatening the opponent, the character and the character's ally flank the opponent. A character gains a +2 flanking bonus on the attack roll. A rogue in this position can also sneak attack the target. The ally must be on the other side of the opponent, so that the opponent is directly between the character and the ally.
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Since the damage bonus can get quite big I wonder how often you can use that sneak attack ?